Utilities and operators of industrial boilers face challenges associated with implementing current and future regulations. In recent years, there has been increasing public and government concern over the environmental impact of nitrogen oxides (NOx) emissions, which contribute to the environmental impact of acid rain. In order to meet the new NOx emission requirements, many utilities install pollution control equipment, using a combination of combustion management and post-combustion processes. Unintended consequences associated with the installation of pollution control equipment have surfaced.
Two approaches are typically used for the control of NOx emissions. These are combustion tuning and post combustion treatment of flue gas. Combustion tuning techniques include low NOx burners, over-fired air systems, reburning, and flue gas recirculation. Post combustion treatments include, but are not limited to, Selective Catalytic Reduction (SCR) and Selective Non-Catalytic Reduction (SNCR).
SCR and SNCR chemically reduce NOx to nitrogen and water. The difference between SCR and SNCR is that SCR utilizes a catalyst which allows the NOx reduction reaction to occur at a lower temperature. The two reagents most commonly used in SNCR systems are either ammonia or urea while SCR uses ammonia.
The generalized reaction when using ammonia is given by:4NH3+4NO+O2→4N2+6H2O
The reaction when using urea is given by:2NO+CO(NH2)2+½O2→2N2+CO2+2H2O
While ammonia has been used successfully to reduce nitrogen oxide emissions, the ammonia is typically introduced in excess of the reaction requirement and is not totally consumed. These fugitive ammonia emissions are called ammonia slip. SCR systems typically operate with ammonia slip values in the 5 ppm range while SNCR systems operate considerably higher. Ammonia slip can be expected to vary widely depending on changes in operating conditions. Some of the slip reports to the gas stream and some to the combustion by-products. Therefore, residues from the combustion process such as fly ash and other combustion by-products will contain ammonia and/or ammonia compounds, such as ammonium sulfate, ammonium bisulfate, ammonium chloride, ammonium hydroxide and ammonium carbonate.
Ammonia slip results in a significant portion of the ammonia compounds being deposited on fly ash. European SCR operation data indicates that combustion of coal in the 6-8% ash range with slip values of 2 ppmv in the flue gas results in concentrations of approximately 100 ppmw as ammonia on fly ash (i.e.—the concentration of the actual compound such as ammonium sulfate is higher, but only the ammonia fraction is of interest, so it is expressed as ppmw of ammonia). Low NOx operations using the SNCR technique commonly produce ammonia concentrations on fly ash in the 1000 ppm range. As more post combustion NOx control systems are placed in operation, increasing amounts of fly ash will contain ammonia.
Requirements for reduction of emissions from coal combustion have often resulted in the coupling of two or more pollution control devices. For example, SCR for reduction of NOx is combined with flue gas desulfurization (FGD or “scrubbers”) for reduction of sulfur dioxide emissions. The coupling of these two devices has resulted in the unintended consequence of increasing the SO3 emissions of the host utility.
During coal combustion, the majority of sulfur in the coal is converted to SO2, with a small percentage of that being further oxidized to SO3. The use of SCR for the control of NOx emissions directs the hot SO2 laden flue gas in the 700° F. temperature range through the SCR system. The vanadium containing catalyst commonly associated with SCR systems also serves as a catalyst for the oxidation of SO2 to SO3. This results in a significant portion of the SO2 in the flue gas being oxidized to SO3. Compounding this problem is the fuel market's current tendency toward utilization of fuels with higher sulfur content which produce higher SO2 concentrations in the flue gas.
Before the SO3 containing flue gas is released into the atmosphere, the flue gas passes through a FGD system which in many cases is inefficient in the capture of SO3. This results in emissions of SO3, the precursor of “Blue Plume”. Blue Plume is formed when SO3 is converted to sulphuric acid (H2SO4) mist. Sulfuric acid is formed as SO3 aerosols cool as they enter the atmosphere and combine with moisture from the ambient environment. Because H2SO4 within a plume, flowing from a stack, is heavier than air, the plume's direction, which was previously upward, changes to an undesirable lateral, or even downward, direction. The visible effect of this phenomena is referred to as “Blue Plume.” Thus, while SCR reduces NOx emissions, it may increase SO3 emissions.
Ammonia can also be used to control Blue Plume. Utilities can inject ammonia into the flue gas stream, usually before the electrostatic precipitator used for control of particulate matter emissions to reduce Blue Plume and visible emissions associated with the SO3.
In this process the injected ammonia reacts chemically with the SO3 in the flue gas producing ammonium sulfate (and possibly small amounts of ammonium bisulfate). The ammonium sulfate combines with the fly ash and is captured by the electrostatic precipitator or other fly ash collection devices. Depending on the SO3 concentration in the flue gas, this process uses ammonia quantities well above any prior art levels and can produce fly ash ammonia concentrations in the 8,000 ppmw range.
The deposition of ammonia and ammonia compounds on combustion by-products such as fly ash can cause problems for its beneficial use. Fly ash has been used successfully for many years in concrete mix designs. The use of fly ash in concrete is the largest single application for fly ash in the United States. In 2002 over 14 million tons of fly ash were used as a replacement for Portland cement in concrete applications. However, excess ammonia concentrations can result in the fly ash becoming unusable due to the odor nuisance and possible worker safety issues. When ammonia containing fly ash is used as a partial replacement for Portland cement, the soluble ammonia compounds contact the wet, alkaline cement matrix which results in the generation of ammonia vapor. Ammonia vapor is produced by the following generalized reaction:NH3++OH−→NH3↑+H2O
Fly ash with high levels of ammonia can be unsuitable for recycling purposes and may cause additional concerns, such as adverse environmental consequences of placing the ammonia laden ash in a landfill. Accordingly, it is important to reduce or remove ammonia and ammonia compounds from combustion by-products such as fly ash prior to their utilization in other applications.
In addition, the concrete industry has placed limits on the amount of ammonia that can be used in fly ash. When fly ash is used at a twenty percent by weight replacement in a concrete mix design (i.e. −20% of the Portland cement is replaced by fly ash), fly ash ammonia levels below 100 ppm are acceptable. When a higher percentage of fly ash is desired in the mix design, the ammonia concentration of the fly ash must be decreased accordingly. In order to use the fly ash in concrete, the ammonia content of the fly ash should desirably be below 60-80 ppm.
Various methods have been used to reduce the levels of ammonia in fly ash. These methods typically involve either adding chemicals to the fly ash, which will then contaminate the fly ash with another chemical while removing the ammonia, or washing the fly ash with water to remove the ammonia (by dissolving the soluble ammonia compounds), producing ammonia-laden water. Another method is described in “Ammonia Removal From Fly Ash Using Carbon Burn-Out”. This paper suggests that fly ash residence times of 45 minutes and temperatures in the 1300° F. range are characteristic of the carbon burn-out process, and that carbon burn-out conditions should be ideal for ammonia removal. The paper reports that tests results indicate that under normal carbon burn-out operating conditions essentially all ammonia was removed liberated from the fly ash material. However, use of the CBO process is only appropriate for certain types of fly ash needing the carbon reduction for which the process was designed.
Therefore, there is still a need for a method of removing the ammonia from a very wide range of ammonia-contaminated fly ash without introducing other chemicals to the fly ash or increasing air or water emissions.